1. Field of the Invention
The present invention relates to both a method and an apparatus for recovering a payload signal from an FSK signal that has been modulated by frequency shift keying wherein, from such signal, a discrete-time series of sampled values are determined for which a phase difference relative to a preceding sampled value is respectively determined.
2. Description of the Prior Art
For transmitting subscriber information such as a call number, via an analog telephone network, frequency shift keying is applied. Such frequency shift keying also is referred to as digital frequency modulation, or FSK. FSK stands for frequency shift keying. Frequency shift keying makes possible the transmission of digital signals by alternating between a frequency above the carrier frequency of a carrier signal and a frequency below this carrier frequency in the rhythm of a rectangular modulation signal. In this way, the required bivalence of the signal is guaranteed.
The method of frequency shift keying is customarily used in a frequency band of 300 Hz to 3400 Hz. The frequencies in this frequency band which lie above or below the carrier frequency of 1700 Hz are referred to as mark frequencies or, respectively, space frequencies and are defined as 1200 Hz or, respectively, 2200 Hz for the USA and as 1300 Hz or, respectively, 2100 Hz for Europe. The rate with which the subscriber information is transmitted typically amounts to 1200 bits/s.
When the transmitted subscriber information is supposed to be processed in a digital PBX connected to the analog telephone network, the payload signal containing the subscriber information, which is impressed on the carrier signal by the frequency shift keying, must be recovered from the modulated signal; that is, the modulated signal that is fed to the digital PBX must be demodulated with the known prior art, it has been impossible to demodulate signals that were modulated by frequency shift keying (henceforth referred to as FSK signals) subsequent to the sampling and digitizing of such signals. The reason for this is that the sampling rate with which the analog signals are sampled in known digital PBXs is too low. The sampling rate of 8000 Hz that is generally used is not suitable for executing the demodulation of the digitized FSK signals in a time-discrete manner using known methods.
For example, one known method provides the utilization of zero crossing discriminators which are intended to determine the zero crossings of the FSK signal and, thus, to make it possible to recover the payload signal. But these zero crossing discriminators are unusable given a sampling rate of 8000 Hz or lower since, in this case, there are too few sampled values present within a sampling period to be able to determine the zero crossings of the FSK signal with sufficient precision. Given a sampling rate of 8000 Hz, one period of the space frequency stated above for the USA contains approximately 3.6 sampled values; that is, 1.8 sampled values per zero crossing. This number of sampled values is insufficient for a reliable demodulation of the digitized FSK signal.
But to increase the sampling rate for purposes of a time-discrete demodulation of the FSK signal would require a significant additional technical outlay which, accordingly, also increases the difficulty with respect to cost. Therefore, the prior art has made use only of analog circuitry for demodulating the FSK signal which has known disadvantages compared to digital circuitry, such as lower transmission reliability due to the increased error rate.
It is therefore, an object of the present invention to formulate both a method and an apparatus which make it possible to reliably demodulate a signal that has been generated by frequency shift keying in a time-discrete manner with a low sampling rate.
The present invention achieves this object by a method in which a discrete-time series of sampled values are determined from the modulated signal and, for the sampled values, the phase difference is respectively determined in relation to a preceding sampled value whose chronological spacing from the sampled value in question is lower than the bit duration within the modulated signal.
The present invention is based on the recognition that the modulated signal represents a pure sine or cosine signal within a sufficiently short time interval. The upper limit of the maximum length of this time interval, that is the greatest possible chronological spacing of the observed sampled value from a preceding sampled value, is defined by the bit duration. The bit duration indicates the length of a signal portion of the modulated signal that is required in order to represent a bit that forms the smallest possible unit of information. The phase difference between two consecutive sampled values whose chronological spacing from one another is defined by such a sufficiently short time interval depends on the frequency that the modulated signal has in this time interval. It is, thus, possible to unambiguously allocate one of the two frequencies that characterize the two possible binary states in the analog signal to the observed sampled value based on the determination of this phase difference. The phase differences that are determined for the individual sampled values can be acquired directly as demodulation signals, thus reproducing the payload signal that has been impressed on the modulated signal by frequency shift keying.
As opposed to the known methods, the present invention makes it possible to demodulate the modulated signal after it has been sampled and digitized. As such, it also makes it possible to use digital circuitry with all its known advantages over analog circuitry. In particular, the inventive method can be applied in a digital PBX in order to demodulate, in a time-discrete manner, the signal that has been received thereby and that subsequently has been sampled and digitized.
The present invention overcomes the problem that is known from the prior art of demodulating a signal that has been modulated by frequency shift keying with a relatively low sampling rate in a time-discrete manner. Specifically, the proposed method is largely independent of the sampling rate, as long as the chronological spacing of the sampled values that are used to determine the above described phase differences does not exceed one bit duration. This aspect of the invention allows a flexible application of the method of the present invention which takes into account the respective technical framework conditions. The present invention can be applied in arbitrary systems in which an FSK signal is to be sampled with a relatively low sampling rate and subsequently demodulated.
In an embodiment of the present invention, a corresponding analytical sampled value is determined from the sampled values wherein the observed sampled value is allocated, on one hand, to the real portion of the analytical signal in unmodified form and, on the other hand, to the imaginary portion of the analytical signal shifted xe2x88x9290xc2x0 in phase. The phase difference for the observed sampled value, thus, can be determined on the basis of the analytical sampled value corresponding thereto and the analytical sampled value that corresponds to the preceding sampled value used to determine the phase difference. Since, in an analytical sampled value such as this, all the phase information is contained in both its real portion and in its imaginary portion, this development of the present invention makes it possible to mask out as it were, the frequency that carries a negative sign, for example, which is usually present in the real sampled value of the sinusoidal cosine-shaped modulated signal in accordance with the rules of Fourier analysis. This procedure specifying the frequency of one operational sign, for instance the frequency that carries the positive sign, permits the unambiguous determination of the phase difference occurring between two consecutive sampled values.
For the observed sampled value, the determined phase difference can be related to a reference phase difference that occurs in a carrier signal on which the modulated signal is based. The amplitude values of the demodulated signal that has been detected according to this embodiment of the method are, thus, divided around the value zero into positive or, respectively, negative values. The time sequence of the two binary states that is to be recovered from the modulated signal thus can be easily identified via positive or, respectively, negative amplitude values of the demodulation signal.
In a further embodiment of the method, the sampled values are fed to a digital filter that determines the real portion and the imaginary portion of the analytical sampled value corresponding to the observed sampled value. The real portion and the imaginary portion of the analytical sampled value are then fed to a phase detection unit that detects the phase difference for the observed sampled value. Advantageously, a non-recursive filter can be used as all-pass filter. This type of non-recursive filter can be implemented as an all-pass filter that provides for the desired phase shift in the prescribed frequency range and that is known in the prior art as a Hilbert transformer.
According to another embodiment of the present invention, a device is provided for executing the method just described. The above-cited technical effects also apply for this device.